Re: [PATCH v2] KVM: Move VM's worker kthreads back to the original cgroups before exiting.

Paolo Bonzini
2022-01-19T18:03:02Z

On 1/18/22 21:39, Tejun Heo wrote:
> So, these are normally driven by the !populated events. That's how everyone
> else is doing it. If you want to tie the kvm workers lifetimes to kvm
> process, wouldn't it be cleaner to do so from kvm side? ie. let kvm process
> exit wait for the workers to be cleaned up.

It does. For example kvm_mmu_post_init_vm's call to
kvm_vm_create_worker_thread is matched with the call to
kthread_stop in kvm_mmu_pre_destroy_vm.

According to Vpin, the problem is that there's a small amount of time
between the return from kthread_stop and the point where the cgroup
can be removed. My understanding of the race is the following:

user process kthread management
------------ ------- ----------
wait4()
exit_task_work()
____fput()
kvm_mmu_pre_destroy_vm()
kthread_stop();
wait_for_completion();
exit_signals()
/* set PF_EXITING */
exit_mm()
exit_mm_release()
complete_vfork_done()
complete();
cgroup_exit()
cgroup_set_move_task()
css_set_update_populated()
exit_notify()
do_notify_parent()
<wakeup>
rmdir()
cgroup_destroy_locked()
cgroup_is_populated()
return -EBUSY
cgroup_exit()
cgroup_set_move_task()
css_set_update_populated()

I cannot find the code that makes it possible to rmdir a cgroup
if PF_EXITING is set.

Paolo